As his name would suggest, Joe was not a meek, quiet fellow. In fact he had such difficulty keeping quiet, both his local and university library banned him. His roommate, being a music major and a sound engineer, said he could solve his problem, but Joe would need to trust him. He gave Joe his 75-watt ghetto blaster, a CD, fake mustache, and a long blond wig. Joe, who is sceptical but intrigued, decided to give it a shot. Joe, disguised as one of the professors, entered the university library. He placed the CD player on the table in front of him; loaded the CD; cranked the volume full and braced himself as he turned it on. Surprisingly there was only a very very faint sshh. Joe then realized that he couldn't hear anything at all. He spoke normally, and then yelled, but no one reacted. Awesome, he thought as he began flailing his arms around and shouting as loud as he could. Unfortunately, the librarian, seeing him moving but hearing nothing, assumed he was choking and performed the Heimlich maneuver on him, knocking his wig off, causing the librarian to perform the hind kick maneuver right out the door. Poor Joe.
But what had his roommate done? What was on that mysterious CD that made everything quiet?

Hint

Answer

Most airline pilots could tell you. Joe's roommate used the same technology that airline and fighter pilots have in their noise cancelling headsets. It works like this; the CD had a low frequency sound (below the 20 Hz which humans can't hear) recorded on it. As long as the volume of the low frequency is louder than your voice or other noise, you hear almost nothing for the low sounds (which humans can't hear) and drown out other sounds. I heard that when this was first discovered that libraries actually used this concept to create quiet until they realized that even though humans couldn't hear the low sound, if loud enough, it would still damage the ears.Hide